Thread: Noise Threshold
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Old 6-Jul-2012, 7:14 AM   #3
GroundUrMast
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Location: Greater Seattle Area
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Yet Another Design Example

Scenario 4a (Low-VHF signal, Roof Top Antenna)

Starting Noise Margin = 18 dB
Additional Low VHF Noise = -13 dB
Antenna gain = +4 dB
Cable loss = -5 dB (long section of RG6)
Receiver Noise Figure = -6 dB
==========
Net residual Noise Margin = 18 - 13 + 4 - 5 - 6 = -2 dB (Failed Design)

Scenario 4b (Low-VHF signal, Roof Top Antenna and Low Noise Preamp)

Starting Noise Margin = 18 dB
Additional Low VHF Noise = -13 dB
Antenna gain = +4 dB
Noise Figure of Preamp = -2 dB
Preamp Gain = 12 dB (Does Not Add or Subtract to/from NM)
Cable loss = -5 dB (long section of RG6) (Overcome by Preamp Gain)
Receiver Noise Figure = -6 dB (Overcome by Preamp Gain)
==========
Net residual Noise Margin = 18 - 13 + 4 - 2 = +7 dB (Possibly Successful Design - There is very little fade margin, only a small bit of additional noise would be expected to disrupt reception.)
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